1. Field of the Invention
This invention relates to an automatic welding machine path correction system and, more particularly, to a path correction system in an automatic welding machine for performing welding while weaving a welding torch left and right with respect to a welding line.
2. Description of the Related Art
An automatic welding machine performs welding by applying a voltage across a wire and a workpiece to produce an arc at the tip of the wire, causing the wire and the workpiece to be melted by the heat generated by the arc, and moving the wire tip along a welding path while the wire is successively paid out in small increments, thereby welding the workpiece. FIG. 8 is a schematic view of such a welding machine. In FIG. 8 wire WR is paid out in small increments in the direction of the arrow by feed rollers FR via a guide member GB so that its tip protrudes from the end of a torch TC. The rate of feed of the wire WR is limited so that the tip of the wire will be spaced a predetermined distance from the surface of the workpiece WK. A welding power supply PS, which applies the voltage across the wire WR and the workpiece WK continuously generates a high voltage for a predetermined period. The positive potential is applied to the wire WR through the guide member GB, and the negative potential is connected to the workpiece WK. In the torch TC, CO.sub.2 gas is supplied by a gas supply unit, which is not shown. Welding is performed while the CO.sub.2 gas is jetted toward the portion being welded, thereby preventing oxidation of the welded portion.
When a high voltage is generated continuously by the welding power supply PS while the CO.sub.2 gas is fed from the gas supply unit, not shown, and the wire WR is paid out in small increments, an arc is produced at the tip of the wire and both the wire and the portion of the workpiece being welded are melted, fusing the portions of the workpiece together. It has recently become possible to perform such a welding operation automatically by robot. Specifically, the torch TC of the welding machine is grasped by a robot, which moves the torch (the tip of the wire) along a welding path to weld the workpiece.
When weaving is performed along a welding line CT, as shown in FIG. 6, the robot control unit moves the torch TC upon calculating the extreme points to the left and right from starting and end points, which have been taught, as well as amplitude and frequency of the weaving pattern. However, in cases where there are differences in the machined precision of the workpiece or where there is a shift in the location to be welded, welding cannot be performed accurately at the predetermined location of the workpiece when the torch TC is moved based on the taught points. Accordingly, as shown in FIG. 7, the conventional practice is to monitor the value of the integral of the welding current and effect a path correction upon comparing the integral value with a reference value when the difference between an integral value prevailing at a leftward swing and an integral value prevailing at a rightward swing in one cycle exceeds a stipulated value.
With this method of correcting the path based on the integrated value of the welding current, sensitivity is very low, especially when the current is small, and the method is not suited to some welding modes as far as the welding characteristics are concerned.